When a motor vehicle is exposed to a strong frontal impact, the front end portions of the side rails extending in the longitudinal direction of the vehicle are indeed deformed. In order to dissipate a large portion of the energy of the impact, it is preferable that this deformation occurs in the form of a longitudinal compression, or bunching, of the side rails, in which successive accordion folds form in planes perpendicular to the longitudinal direction of the side rails.
However, during the impact, a side rail commonly folds moving away from its longitudinal axis, a fold forming a vertical rotation axis between two portions of the side rail. In this case, the deformation absorbs a much smaller quantity of energy from the impact. The protection of the remainder of the vehicle, and particularly of the compartment in which the passengers are, is then decreased.
In order to stabilize the bunching of the side rails in the case of an impact, the shape and the size of these side rails are generally selected to confer a large amount of inertia to them, and to increase their transverse stiffness. However, these side rails with reinforced inertia in general have a greater weight and they take up more space than unreinforced side rails. The reinforcement thus generates an increase in the production cost of the chassis frames, an increase in the weight of the vehicle resulting in an increase in its fuel consumption, and a greater size making it more difficult to install the different components of the vehicle around the chassis frame.
Moreover, when the side rails are not sufficiently stiff transversely and risk undergoing deformation due to rotation around a folding line, these side rails are not sufficient to dissipate the energy of a frontal impact, which makes it necessary to consolidate the passenger compartment of the vehicle. The weight of the vehicle, and thus its fuel consumption, are then increased, as is its manufacturing cost.